[delta-audit] bonding: Add stake checkpointing to withdrawFees

[victorges]

What does this pull request do? Explain your changes. (required) This adds a missing checkpoint in BondingManager found during the code4rena audit.

The withdrawFees function was using autoClaimEarnings but not autoCheckpoint modifiers, so fixed that and added a test.

Specific updates (required)

• Add autoCheckpoint modifier to withdrawFees
• Add unit test to make sure checkpoint is made

How did you test each of these updates (required) yarn test to be deployed to devnet

Does this pull request close any open issues? Fixes:

• https://github.com/code-423n4/2023-08-livepeer-findings/issues/104
• https://github.com/code-423n4/2023-08-livepeer-findings/issues/86

Checklist:

• [x] README and other documentation updated
• [ ] All tests using yarn test pass